Currently, because a quadrature amplitude modulation (QAM) technology can provide higher spectrum utilization efficiency, currently the quadrature amplitude modulation technology has already been widely applied to various digital communications fields. However, compared with quadrature phase shift keying (QPSK) modulation used in current 100 G transmission, higher order QAM modulation has more amplitudes and phases, resulting in increased envelope fluctuation of a modulated signal, so that nonlinear effects of a signal during data link transmission are enhanced and transmission performance of an entire system is affected. In addition, because the higher order QAM modulation has more phases than the QPSK modulation, when a receiver performs phase retrieval on a received signal, cycle skipping is more frequent, thereby affecting system performance.
For the foregoing content, in the prior art, a signal sending and receiving method of offset-quadrature amplitude modulation (Offset-QAM) is generally used to resolve the foregoing problem. Specifically, a transmit end transmits a signal by dividing the signal into multiple subcarriers, and when each subcarrier is transmitted, a quadrature electrical signal and an in-phase electrical signal are offset by half a baud period and then an optical subcarrier is modulated. In this case, although an envelope fluctuation degree of a signal is reduced to some extent, a signal spectrum is not essentially improved, and a cycle skipping effect caused when a receiver performs phase retrieval on a received signal cannot be improved either.